The present invention relates to a system for the electronic commutation of a brushless DC motor having three phase windings which are electrically displaced by 120xc2x0, comprising a semiconductor bridge of six power semiconductors, which drives the phase windings for generating a rotating magnetic stator field, a control unit which correspondingly drives the power semiconductors, and a device for detecting the respective rotational positions of a rotor exhibiting a permanent-magnetic magnet wheel, the device for detecting the rotor position being constructed as sensor-less evaluating unit, in such a manner that the voltage induced by the rotating magnet wheel, which can be measured in each case at the phase winding terminal which is currently not driven, is detected and evaluated.
For the electronic commutation of collector-less DC motors, the control unit driving the semiconductor bridge must, in principle, be supplied with information on the current rotational position of the permanent-magnetic rotor relative to the stator so that the respective suitable phase windings can be driven for generating a torque in the desired direction of rotation to generate by this means an optimum rotating stator field. In general, the rotor position is detected by rotor position sensors, especially Hall sensors by means of the permanent-magnetic rotor field. In many cases, however, the motor and its associated commutation electronics must be spatially separated from one another so that electrical connecting lines are required via which, on the one hand, current is supplied to the phase windings, and on the other hand, the signals of the rotor position sensors are transmitted to the control electronics. The connecting lines and the necessary connections, e.g. plug-in connections, however, lead to very high cost expenditure, especially for production (assembly) and material and in addition also increase the susceptibility to faults.
In contrast, a sensor-less detection of the rotor positions is provided in systems of the generic type as a result of which (Hall) sensors and corresponding separate line connections can be omitted. Instead, the polarities or polarity changes (zero transitions) of the voltages induced in the motor windings by the permanent-magnetic rotor field (internal voltage or, respectively, EMF=electromotive force) are detected via the respective non-driven currentless winding terminal and evaluated.
Such a sensorless commutation system is known, for example, from EP 0 881 761 A1. In this arrangement, the voltages at the three motor winding terminals are detected by an EMF detector circuit, and from this three corresponding binary output signals are generated for the sensorless rotor position determination. In this manner, six different combinations of these output signals are generated over one electrical revolution of the rotor which makes it possible to determine the rotor position in 60xc2x0 segments. Each combination or each rotor position segment, respectively, is associated with one switching state of the bridge semiconductors (one semiconductor being pulsed in a pulse-width modulated manner for setting the speed in the respective switching state). As a result, there are six different switching states at 60xc2x0 intervals over one electrical rotation. In this arrangement, the stator excitation (stator field) is commutated in six steps within one electrical rotation so that this is a 6-step commutation.
Similar commutation circuits with 6-step commutation are also described in documents EP 0 872 948 A1, DE 39 34 139 C2, DE 33 06 642 C2 and DE 36 02 227 A1.
In U.S. Pat. No. 5,491,393, a commutation control for brushless DC motors is described, this also being a 6-step commutation in principle because one electrical rotation of the motor is divided into six basic steps with respect to the changing direction of stator excitation. Although each of these basic steps is subdivided into two sections, but between these there is only one change of the power semiconductor which is in each case pulsed in a pulse-width modulated manner. In each step, however, only two of the six semiconductor switching elements of the power bridge circuit are in each case active so that it is always only two of three winding terminals which are actively connected to the positive or negative terminal of the DC source via the switching elements. For this reason, there can be only six different directions of stator excitation so that this is clearly a 6-step commutation in the proper sense.
A prior art which is very similar to U.S. Pat. No. 5,491,393 is also disclosed in U.S. Pat. No. 5,835,992, according to which six times two switching states or switch combinations are also provided but there, too, the direction of the stator excitation is changed in only six steps (=6-step commutation).
The present invention is based on the object of creating a system of the generic type initially described, by means of which a reduction in running and commutation noises is achieved whilst maintaining the inexpensive and fault-insensitive sensorless rotor position detection.
According to the invention, this is achieved by the fact that the control unit drives the power semiconductors in twelve different switching states by means of a 12-step commutation beyond one electrical revolution of the DC motor in dependence on the rotor positions. This 12-step commutation according to the invention is effected in such a manner that each of these twelve switching states causes a different excitation state of the stator relating to the direction of the magnetic field generated in it. For this purpose, it is provided that, on the one hand, switching states in which two power semiconductors of the semiconductor bridge are driven and, on the other hand, switching states in which three power semiconductors are driven, continuously occur in an alternating manner. In contrast, the power semiconductors in a 6-step commutation are driven in switching states which exclusively lead to six different excitation states of the stator. In the 12-step commutation, the stator excitation is in each case commutated by smaller angular steps than in a 6-step commutation as a result of which a continuously circulating stator field is generated.
The invention is initially based on the finding that the application of a 6-step commutation leads to motor noises, the xe2x80x9ccommutation clickingxe2x80x9d, due to an abrupt change in the stator excitation during the commutation time. When the electric motor is used as drive unit for a ventilator or a blower, these commutation noises are very pronounced and disturbing, especially in the lower speed range in which the flow noises recede, and cannot, therefore, be accepted in many applications. According to the invention, by comparison, a clear reduction of these noises can be achieved (especially in motors with an external rotor) in that, instead of only six switching states, twice the number of switching states of the power semiconductors of the semiconductor bridge are now provided per electrical revolution, i.e. the stator excitation is commutated in twelve steps instead of six steps within an electrical angle of rotation of 360xc2x0.
Although such a 12-step commutation is known per se, this is, however, exclusively with separate sensors for detecting the rotor position. The reason for this is that, among experts, it has hitherto always been assumed that sensorless construction of a 12-step commutation is not possible because sensorless rotor position detection always presupposes that in each case one winding terminal is currentless, that is to say isolated from the DC source in order to be able to detect induced internal voltage (EMF) at all with a sensorless evaluating unit. In distinction from the 6-step commutation, however, this is not generally so with 12-step commutation because there are areas of winding current overlaps in which all three winding terminals carry current.
Thus, the present invention is based on the further finding that 12-step commutation is only possible if care has been taken that polarity changes or zero-transitions of the internal voltage occur precisely in those intervalsxe2x80x94severely shortened in the case of 12-step commutationxe2x80x94in which the corresponding winding terminal is in each case isolated from the DC source and can therefore be used for detecting the control-related magnitudes.
In real terms this is preferably achieved by the fact that EMF evaluating unit in each case detects and evaluates the internal voltage between a motor winding terminal and a common reference point. The reference point can here be either the actual neutral point of the motor, brought out via a line, if the motor windings are connected in a star connection, or an equivalent neutral point. It should be noted in this respect that, of course, a motor delta connection can also be fictitiously transferred into a star connection. According to the invention, the EMF evaluating unit accordingly, in practice, detects the respective phase voltage or xe2x80x9cphase EMFxe2x80x9d in the star system instead of the phase-to-phase xe2x80x9cconductor EMFxe2x80x9d which can be measured between two winding terminals and which, by comparison, is phase-shifted by 30xc2x0. It is due to this measure, according to the invention, that zero transitions of the internal voltage can be detected at all because they fall precisely into the steps in which a winding terminal is in each case currently currentless over a short range of 30xc2x0. In contrast, detection and evaluation of the voltage which is in each case between two motor winding terminals would not be suitable for 12-step commutation because, in this arrangement, the zero transitions of the induced internal voltage would always occur when all three winding terminals are carrying current, i.e. are connected to the DC source so that detection would not be possible at all.